Question about power tubes and Ot's

[lo-watt]

So i want to build a low powered tweed twin in the style of the Cesar Diaz modded Twin that Clapton used for a long time. Basically,it was a low powered Twin that had an early Showman PT added and had 4 6L6's and a SS rectifier(diodes). From the pictures Ive seen the rest of the amp looks pretty stock(other than filter caps/values) and may well have the original OT. Removing tubes has long been discussed in big amps(BFTR),but how does ADDING power tubes change the amp,how it works and what it needs to see from the speakers? Would i need a 2 ohm load from the pair of 12's now? Will a stock LPTT OT be able to keep up with 4 power tubes? What biasing mods would be needed if any? or any other mods? Thanks in advance,Kenny

[loudthud]

The info you presented is confusing. Do you have any pictures or links to other discussions? Is it really built on a Low Power Twin chassis (5 knobs) or the High Power Twin (6 knobs) chassis ? Are you sure it has four 6L6 tubes and not two 6L6 and two 5U4 or other rectifiers.

[lo-watt]

The pictures are at the Bonhams website from the Crossroads auction where the amp was sold. Yes,its a low power TT made in 57. Yes it has four Sylvania 6L6's and diode rectifier. The auction has interviews with Diaz where he details what he did to it. But he doesnt mention the OT. Which could very well be stock LPTT.

[Leo_Gnardo]

Some 25ish years ago I had a customer bring an a tattered Twin amp, 2 bulbous 5U4 and 2 6L6. He wanted it converted to solid state power supply, and 4 x 6L6GC. I warned him, unless I change the OT, he wouldn't attain max power unless he ran a 2 ohm speaker load. When all was said & done I measured same power to the speakers as before. No matter, he swore it sounded better. And played the dickens out of that amp touring around the eastern half of the USA for 10 years or so. Then another customer bought that amp, and had me convert it back to stock. Whew... I think he still has it. Only "improvement" now, it has a new baffle, and travels in safety & style enclosed in a proper road case.

[lo-watt]

So a Bassman OT would be right? 2 ohm load?

[pdf64]

Bear in mind that 4 x 6L6 may overly stress the heater winding of a PT originally specified for 2 x 6L6.
Leo, how did you manage that?

Note that if the OT primary impedance is changed to a lower, 4 x 6L6 appropriate value, then the PT's HT winding may get stressed too, from the additional current draw at high signal levels.

[lo-watt]

Keep in mind that according to the interview,Cesar Diaz used an Early Showman power trans built for 4xpower tubes AND no 5v winding for a tube rectifier. So im mostly concerned with the OT. that PT should be able to handle it just fine.

[Leo_Gnardo]

So a Bassman OT would be right? 2 ohm load?

The point was, if you double the number of power tubes driving the OT, you halve the optimum output impedance of the OT, assuming you retain the original OT. Theoretically anyway. In practice just about right BUT you don't gain the advantage of doubling output power the way Johnny Guitar hopes it would be... SO it would have been appropriate to replace the OT with a Twin or similar OT, that happily drives 4 ohms - the actual speaker setup in the amp. Because the amp's owner either is too cheap to pay for a proper OT, OR wants to retain the "originality" of the amp despite having had a major mod done on its output amp & power supply (go figure), OR ignorance, "stop confusing me with scientific reality" attitude, the OT never did get changed. In a surprising turn of events the owner of the amp that had me modify it now works for a major LA area music store, repairing amps and building custom amps to order. Ain't that somethin'???

[lo-watt]

I just feel that Cesar used the original OT and let it ride. It might’ve been what gave the amp it really awesome sound.

[Leo_Gnardo]

I just feel that Cesar used the original OT and let it ride. It might’ve been what gave the amp it really awesome sound.

Here's what gave that amp its awesome sound:



Them ten fingers right there.

[g1]

Putting a showman PT in a low power twin means it is no longer low power. That's why Fender had to come up with a bigger OT for the showman.
If Cesar left the LPTT low power OT in the now higher power (showman PT) circuit, I doubt it would have lasted long.
Put in simplest terms, your question is "can I put a 50W OT in a 100W amp?".

[Chuck H]

Putting a showman PT in a low power twin means it is no longer low power. That's why Fender had to come up with a bigger OT for the showman.
If Cesar left the LPTT low power OT in the now higher power (showman PT) circuit, I doubt it would have lasted long.
Put in simplest terms, your question is "can I put a 50W OT in a 100W amp?".

Pretty much. But the wattage isn't a certainty as it depends on voltage. And the capability of the OT might be flexible enough? But I do think you're right that a Showman PT is going to provide enough voltage that running it's intended four tube compliment it's a bad idea to keep a 50W OT in the amp. Maybe if a lower voltage PT were used? But now it's just getting silly. This whole thread, from the perspective of the OP, seems to be more about chasing an indefinable tonal adjunct than anything else. It's already been implied that doing something "wrong" might be a magical tone maker for this design. And yet there seems to be some will to change it, but keep it the same? I've been following, but I just can't get a coherent bead on the actual subject or question yet.

[lo-watt]

Pretty much. But the wattage isn't a certainty as it depends on voltage. And the capability of the OT might be flexible enough? But I do think you're right that a Showman PT is going to provide enough voltage that running it's intended four tube compliment it's a bad idea to keep a 50W OT in the amp. Maybe if a lower voltage PT were used? But now it's just getting silly. This whole thread, from the perspective of the OP, seems to be more about chasing an indefinable tonal adjunct than anything else. It's already been implied that doing something "wrong" might be a magical tone maker for this design. And yet there seems to be some will to change it, but keep it the same? I've been following, but I just can't get a coherent bead on the actual subject or question yet.

Ok, let me try to be more clear: I want to build a clone of Eric Claptons 57 Low Power Twin that was modified by Cesar Diaz in 1986. Cesar installed a Showman Power Transformer to allow the use of 4 6L6's and a diode rectifier. Much of the rest of the amp was left stock. The OT may still be stock. The speakers were reconed Fender Oxfords from the 70's,so they may be 4 ohm each or 8 ohm each. Thats also unknown. Bottom line:if i use a Tweed Bassman transformer with the Blond Showman Power Transformer,will it live?

[g1]

The question is how long will it live?
I certainly would not use a bassman OT as a replacement in a blond Showman (6G14), so I wouldn't use that transformer set in a build either.
You weren't planning on dropping the B+ with zeners or something were you? That would change things.

[lo-watt]

Eric said the amp got hot a lot but never died in 20 years. I can live with that. I understood the tweed Bassman OT was similar to the tweed Low Power Twin OT, but I may be mistaken.